Cellulose and hemicellulose are the most abundant plant materials produced by photosynthesis. They can be degraded and used as an energy source by numerous microorganisms (e.g., bacteria, yeast and fungi) that produce extracellular enzymes capable of hydrolysis of the polymeric substrates to monomeric sugars (Aro et al., J. Biol. Chem., 276: 24309-24314, 2001). As the limits of non-renewable resources approach, the potential of cellulose to become a major renewable energy resource is enormous (Krishna et al., Bioresource Tech., 17: 193-196, 2001). The effective utilization of cellulose through biological processes is one approach to overcoming the shortage of foods, feeds, and fuels (Ohmiya et al., Biotechnol. Gen. Engineer Rev., 14: 365-414, 1997).
Cellulases are enzymes that hydrolyze cellulose (beta-1,4-glucan or beta D-glucosidic linkages) resulting in the formation of glucose, cellobiose, cellooligosaccharides, and the like. Cellulases have been traditionally divided into three major classes; endoglucanases (EC 3.2.1.4) (“EG”), exoglucanases or cellobiohydrolases (EC 3.2.1.91) (“CBH”) and beta-glucosidases ([beta]-D-glucoside glucohydrolase; EC 3.2.1.21) (“BG”) (Knowles et al., TIBTECH 5: 255-261, 1987; and Schulein, Methods Enzymol., 160: 234-243, 1988). Endoglucanases act mainly on the amorphous parts of the cellulose fiber, whereas cellobiohydrolases are also able to degrade crystalline cellulose (Nevalainen and Penttila, Mycota, 303-319, 1995). Thus, the presence of a cellobiohydrolase in a cellulase system is required for efficient solubilization of crystalline cellulose (Suurnakki et al., Cellulose, 7: 189-209, 2000). Beta-glucosidase acts to liberate D-glucose units from cellobiose, cello-oligosaccharides, and other glycosides (Freer, J. Biol. Chem., 268; 9337-9342, 1993).
Cellulases are known to be produced by a large number of bacteria, yeast and fungi. Certain fungi produce a complete cellulase system capable of degrading crystalline forms of cellulose, such that the cellulases are readily produced in large quantities via fermentation. Filamentous fungi play a special role since many yeast, suck as Saccharomyces cerevisiae, lack the ability to hydrolyze cellulose (see, e.g., Wood et al., Methods in Enzymology, 160: 87-116, 1988).
The fungal cellulase classifications of CBH, EG and BG can be further expanded to include multiple components within each classification. For example, multiple CBHs, EGs and BGs have been isolated from a variety of fungal sources including Trichoderma reesei (also referred to as Hypocrea jecorina), which contains knows genes for two CBHs, i.e., CBH I (“CBH1”) and CBH II (“CBH2”), at least eight EGs, i.e., EG I, EG II, EG III, EGIV, EGV, EGVI, EGVII and EGVIII, and at least five BGs, i.e., BG1, BG2, BG3, BG4, BG5 and BG7 (Foreman et al. (2003), J. Biol. Chem. 278(34):31988-31997), EGIV, EGVI and EGVIII also have xyloglucanase activity.
In order to efficiently convert crystalline cellulose to glucose the complete cellulase system comprising components from each of the CBH, EG and BG classifications is required, with isolated components less effective in hydrolyzing crystalline cellulose (Filho et al., Can. J. Microbiol., 42:1-5, 1996). Endo-1,4-beta-glucanases (EG) and exo-cellobiohydrolases (CBH) catalyze the hydrolysis of cellulose to cellooligosaccharides (cellobiose as a main product), while beta-glucosidases (BGL) convert the oligosaccharides to glucose. A synergistic relationship has been observed between cellulase components from different classifications. In particular, the EG-type cellulases and CBH-types cellulases synergistically interact to efficiently degrade cellulose.
Although beta-glucosidase compositions have been previously described, there remains a need for new and improved beta-glucosidase compositions. Improved beta-glucosidase compositions find use for example in saccharifying biomass. Beta-glucosidases that exhibit desirable levels in one or more of expression, activity and stability are of particular interest.